The invention relates to an electromechanically and hydraulically operable motor vehicle brake having an actuator assembly, comprising: a brake housing, an actuation member which is movable in relation to the brake housing for hydraulically or electromechanically moving a brake lining, a motor drive, a displacement mechanism arranged between the motor drive and the movable actuation member, a gear train assigned to the displacement mechanism, and a separate self-locking device, which is designed to block the displacement mechanism as needed, wherein the gear train has at least two gear stages.
For a long time, motor vehicle brakes have been in use, in which the brake linings are hydraulically displaced in the usual manner in the case of normal operational braking while driving the vehicle, but in which for activation of a parking brake function, an electromechanical displacement of the brake linings or at least an electro-mechanically triggered locking of the brake linings in a braking position takes place. Such motor vehicle brakes offer the advantage that the parking brake function can be activated or released more conveniently by simple operation of an activation switch.
Such electromechanically and hydraulically operable motor vehicle brakes are known from the state of the art.
WO 2008/037738 A1 thus describes a motor vehicle brake that can be operated both hydraulically and electromechanically. In a normal operational situation, i.e., while driving the vehicle, this motor vehicle brake is operated hydraulically in the usual way. However, the electromechanical operating function is activated for activation of a parking brake. This triggers an electric motor, driving a spindle-nut arrangement via a displacement mechanism with a gear train. The gear train is designed to be self-locking with a worm gear to prevent the parking brake effect from being diminished when the parking brake is activated. However, the self-locking effect has the disadvantage that only very low degrees of efficiency can be achieved, so that the components, in particular the electric motor, must be designed to be relatively powerful and must have a high power consumption. It is true that the total efficiency of the system is comprised of the product of the individual efficiencies of the components. For example, efficiency is derived from the product of the efficiency of the motor, the efficiency of the downstream gear train and the spindle-nut arrangement. Thus, an efficiency in the range of only 30% or less can be achieved with a gear train having a self-locking effect.
DE 10 2012 208 294 A1 describes a motor vehicle brake, in which a separate self-locking device is arranged directly on the nut-spindle assembly. Reference is made particular to FIG. 8 in this document, where it is shown that the self-locking device, which is designated as coupling device 41, is arranged between the rolling body ramp gear and the brake piston. Such a direct spatial correlation of the self-locking device with the spindle-nut assembly and a ball screw drive has the disadvantage that the reactive forces occurring in activation of the brake and maintenance of a (parking) brake state must be absorbed by the self-locking device to the full extent. The self-locking device must be designed to be solid accordingly, as a result of which it takes up a substantial amount of space. Therefore, the brake must also be designed to be disproportionately large in the axial direction, in particular with respect to the threaded spindle.
Furthermore, DE 10 2011 102 860 A1 discloses a motor vehicle brake that has been improved in comparison with this prior art, in which the self-locking device is arranged close to the spindle-nut arrangement but is integrated into the gear train, so that the installation space can be reduced in the axial direction with respect to the threaded spindle. This brake can be designed to be more compact.
There is basically also a problem in diverting the reactive forces occurring in the self-locking device. One thing to be taken into account here is that as many housing components as possible should be manufactured from a lightweight material such as aluminum, for example, in order to reduce their weight. However, this has the disadvantage that it is very difficult to divert high reactive forces such as those possibly occurring with self-locking devices of the type described above and they may be subject to high wear.
Further prior art is to be found in the documents DE 103 49 078 A1, DE 10 2008 054399 A1, KR 101 331 783 B1 and U.S. Pat. No. 5,829,845 A.